Thermal Mass (Thermal Dispersion) Flow Meters and Switches

How they work, where they excel, advantages and limitations.

How they Work:

Thermal mass flow meters are also known as thermal dispersion or calometric flow meters. They operate on the principal that a flow stream produces a rate of heat transfer, where heat is removed and cooling takes place, that is proportional to the mass flow of the media. There are two elements to a typical thermal mass flowmeter, a heating element and a sensing element. The heating element is cooled by the flow and is detected by the sensing element. This difference is translated into a flow value.

To envision how this works, imagine you are standing outside in a t-shirt on a fall day. There is a light breeze blowing, it’s about 60 degrees and you are comfortable. Now imagine that the breeze turns into a strong wind. The increase in the wind speed begins to chill you because your skin is loosing more heat. This is the same principle that thermal mass flow meters operate on. The stronger the wind, the less heat you retain. With thermal dispersion measurement; the more flow, the less able the probe in the flow stream is able to retain heat.

Advantages:

• Compatible with many gases
• Can be used for saturated gases
• Accurate and repeatable over wide range
• No moving parts to wear or break
• Less maintenance
• Long operational life
• Designed for heavy industrial use
• High flow rate sensitivity
• High turndown, up to 100:1
• More economical than other mass flow technologies
• Unaffected by changes in pressure, temperature, or density
• Quick reaction time to changes in flow
• Lower installation cost, no extra instrumentation needed
• Insertion types can be used in large pipes

Considerations:

• Primarily used for laminar-flowing gas in closed systems
• Straight pipe runs can be reduced using dual plate flow conditioning
• Two styles, insertion and inline

Examples of Gases Measured:

• Compressed Air
• Carbon Dioxide
• Argon
• Nitrogen
• Oxygen
• Natural Gas
• Biogas
• Air

Common Applications for Gas:

• Oil and Gas, Upstream and Downstream
• Refineries
• Petrochemical
• Powerplants
• Nuclear Plants
• Clean Energy
• Landfill Gas Recovery, Biogas
• Flare Gas
• Stack or Flue Gas
• Aeration
• Wastewater Treatment
• Chemical Operations
• Food and Beverage
• Mining and Metals
• Manufacturing
• HVAC
• Pharmaceuticals
• Marine
• Consumption and Supply Monitoring
• Leak Detection
• Monitoring Distribution Networks
• Mixing and Blending of Gas
• Burner or Boiler Feed Control

Limitations:

• Calibration is specific to a particular gas or mixture and accuracy decreases if changed
• Typically for clean, non-abrasive media
• Media cannot have moisture or droplets
• Thermal properties of media must be known
• Initially, the costs are higher